ONLY a OEM jeep/mopar thermostat will work, I had the same exact trouble on mine when I first bought it.
I had a custom 3 row copper core radiator built,before I got an OE stat. The oe stat has a check ball for air bleeding, it alone dropped the temp 25 degrees.
At the xler dealership they call it the "heavy duty stat".

Let us know if that helps.

SD